Peter the Great was designing the city as another Amsterdam and Venice, with canals instead of streets and citizens skillful in sailing. Initially, there were only about ten bridges constructed in the city, mainly across ditches and minor creeks. By Peter's plans, in the summer months, the citizens were supposed to move around in boats, and in the winter months when the water froze to move in sledges.
http://en.wikipedia.org/wiki/Neva_River
See also:
http://www.panoramio.com/photo/31387096